Continuous casting of steel has conventionally been carried out generally by a continuous casting machine either of the curved-mold or the straight-mold type. From among said two types, a continuous casting machine of the curved-mold type is shown in FIG. 1. As shown in FIG. 1, a molten steel 1 poured in a tundish 2 is teemed through a submerged nozzle 3 into a mold 4, where the molten steel is cooled to form a thin solidification shell 5. The molten steel having thus formed the solidification shell is withdrawn through a group of support rollers 6, a group of guide rollers 7, a group of reduction rolls 8 and a group of pinch rolls 13, arranged in this order below said mold 4. In the meantime, said solidification shell 5, cooled by cooling water sprayed from a number of nozzles 9 arranged between said rollers and rolls, has a gradually increasing thickness, whereas a part of steel still in the molten state 10 (hereinafter called "crater") has a gradually decreasing thickness, and finally the solidification of molten steel is completed. A continuously cast strand 11 having a prescribed crosssectional shape is formed by cooling said molten steel through the adjustment of the cooling rate, the withdrawal speed and other factors, so that the top of said crater 10 where the solidification of molten steel is completed may be in said group of reduction rolls.
However, in cast strands, whether cast by a continuous casting machine of the above-mentioned curved-mold type or of the straight-mold type, there often occur such inner defects as a center porosity (an assembly of fine shrinkage holes produced at the center of a cast strand), a center pipe (a center porosity growing up into a large pipe-shaped shrinkage hole) and a center segregation.
The above-mentioned inner defects in a cast strand are considered attributable to the following:
1. The solidification shell of a cast strand near the crater top mechanically bulges outward under the effect of the static pressure of molten steel at the crater top (the height from the molten steel surface in the mold to the crater top, multiplied by the density of molten steel), and steel still in molten state with condensated impurities and constituent elements is enclosed in this bulging, thus causing segregations.
2. Steel still in molten state with condensated impurities and constituent elements in the cast strand near the crater top is pulled by the part of steel having already solidified under the sucking effect occurring on the solidification and shrinkage. If, however, the transfer of this part of steel still in molten steel is insufficient, not only pores but also center segregations are formed in this part of the cast strand.
For the purpose of eliminating inner defects in a cast stand occurring from the aforementioned causes, a method has conventionally been proposed, for reducing a cast strand by at least 10% near the crater top through reduction rolls. This conventional method permits removal of center porosities and center segregations to some extent. According to this conventional method, however, the high draft such as at least 10% causes many inner cracks on the liquidus-solidus interface of the molten steel near the crater top. Furthermore, because of the steel still in molten state with condensated impurities and constituent elements penetrating into these inner cracks, the occurrence of segregations is still eradicable, and a satisfactory effect for the improvement of the inner quality of a cast strand has not as yet been obtained.